1. Field of Invention
The invention relates to a heatable mirror having a specifically designed heat-conducting layer made from conductive particles, a process for applying heat-conducting layers to substrates and the use of this process for producing heatable mirrors.
2. Description of the Related Art
Heatable mirrors having a heat-conducting layer made from conductive material are known. Hence a heatable mirror is described in U.S. Pat. No. 3,686,437 in which a heat-conducting layer has been applied to a glass substrate under vacuum or by sputtering. This United States patent proposes the use of a nickel-chromium alloy as the heat-conducting layer.
The disadvantage of this mirror is that firstly, the technologies for applying the heat-conducting layer are very complex. As described in the United States patent, application of the nickel-chromium alloy should be carried out either under vacuum or using a sputtering process. Both process steps are too complex and too expensive for mass production. Furthermore, it has been shown that a mirror of this type obviously also does not have adequate heat capacity due to the only very thin layers. This leads to the time span for keeping a mirror free of condensation being too long. Also it is not possible using mirrors of this type to heat certain areas specifically more strongly than others, which is required [is] in many cases because of the construction of the mirror.
The object of the present invention is therefore to propose a heatable mirror, the heat-conducting layer of which permits high heat capacity and any design of layout for the heat-conducting layer. The object of the invention is also to provide a cost-effective and simple process for mass production by means of which a heat-conducting layer may be applied to a substrate.
[The object is achieved with regard to the heatable mirror by the features of patent claim 1 and with regard to the process by the features of patent claim 17. The sub-claims show advantageous further developments of the solution according to the invention.]
It is thus proposed according to the invention that the heat-conducting layer is formed by conductive particles at least partially embedded in the surface of the substrate and/or the reflective layer. It is thus essential for the heatable mirror of the invention that the particles have an average diameter of 3 to 100 xcexcm. FIGS. 4-6 show scanning electron-microscope images in 30, 100 and 1,000 time magnification, from which particle formation can be seen. Accordingly, the heat-conducting layer has a xe2x80x9crough surface:. FIG. 6 in particular shows that the particles have obviously only been changed insignificantly by the application process. FIGS. 4 to 6 also show that the particles have partially penetrated into the surface of the substrate, here a mirror. Adhesion of the particles to the substrate is thus essentially produced by the penetration of particles into the surface of the substrate. Of course partial sintering of the particles with one another and with the surface also takes place. It is important for the heatable mirror having the heat-conducting layer as described above, that a certain roughness, that is an average particle diameter or 3 to 100 xcexcm is maintained, and that a thus cohering conductive layer is produceD. The heat-conducting layer of the mirror of the invention thus differs from those which are known hitherto from the state of the art, which have been applied conventionally via sputtering processes or evaporation methods. In all these cases a very finely divided homogeneous conducting layer is thus produced which has the disadvantages of the state of the art outlined in the introduction.
The particles themselves are preferably selected from the metals Al, Zn, Sn, Cu, Ni and/or alloys thereof. It is most particularly preferred for the heatable mirror of the invention if the particles consist of aluminum and/or an aluminum alloy having at least 96% aluminum. A further preferred embodiment of the mirror of the invention is characterized in that the substrate for the heatable mirror is glass. The material pairing glass and aluminum and/or aluminum alloy in particular as particles has proved to be particularly superior in its properties with regard to the mechanical adhesion and electrical heat capacity.
For the mirror of the invention, the heat-conducting layer may be designed to be both over the entire surface or strip-like. For the embodiment in which a strip-like heat-conducting layer is present, it is thus important that the regions between the heat-conducting strips consist of an insulating separating layer. This insulating separating layer is preferably a thin film having a width of 0.2 mm to 1 mm. The minimum width is important because otherwise bridging may occur during spraying. The separating layer is produced from a formulation containing resins and solvents. Preferred resins are thus shellac, gum arabic or colophony. The insulating separating layer may thus also contain further additives, as known per se from the state of the art. This separating layer is applied; in the required form by spreading-on, spraying or brushing.
It has also proved to be advantageous if the layer thickness of the heat-conducting layer lies in the range from 10 xcexcm to 100 xcexcm, preferably in the range from 40 to 60 xcexcm. A particular advantage of the heatable mirror of the invention is that the layer thickness of the heat-conducting layer may be selected to be different, so that increased heat capacity is then obtained at particularly critical points in the mirror area, so that condensation is also prevented there. For the case that strip-like heat-conducting layers are used, the strip widths of the heat-conducting layer are 2-20 mm. Also the strip width of the heat-conducting layer may be selected to correspond to the width so that a wider strip width is selected at particularly critical points at which dehumidification of the mirror is only possible with difficulty. Of course the invention also includes all embodiments in which both the layer thickness of the layer and also the strip width is varied.
The heat-conducting layer of the heatable mirror of the invention also shows considerable improvement compared to the state of the art apart from the advantages described above. It has also been shown to the expert in a manner which could not be foreseen, namely that the heat-conducting layer [as defined in patent claim 1] may be provided with a polymer coating which serves as chip protection, for thermal and electrical insulation as well as also as adhesive for fixing the mirror frame.
It was hitherto namely conventional in the state of the art to apply chip protection which was present in the form of adhesive films or foils and which had to be produced in a separate working step and then adhered to the mirror. In the mirror of the invention it is now possible to apply a polymer coating very simply, for example by spraying, particularly because of the surface formed by the particles. It has thus proved to be particularly advantageous if as the polymer coating such a one is selected which consists of a self-curing silane-modified polymer. This now produces the advantage that the mirror is coated over the entire surface with the polymer on the side on which the heat-conducting layer is applied in a single working step, and that simple curing in air then takes place. Silane-modified alkyd resin systems are preferably used as polymers of this type. They are characterized particularly in that they are single-component systems and cross-link in air moisture to give a flexible product. A further advantage of such a coating is that this coating acts as a steam barrier and is UV stable and functions as a sealing material with very good workability.
In the heatable mirror of the invention provision is made, as already known from the state of the art, in that contract points are present for contacting with a voltage source. These contact points are thus preferably designed so that they are connected to the heat-conducting layer via an additional metal layer. This additional metal layer is preferably selected from the metals Zn and/or Sn. This additional metal layer thus has a thickness of 50 to 100 xcexcm.
The layers known from the state of the art may be used as the reflective layer for the mirror. Examples of these are dichroic layers or chromium or silver layers.
The invention also relates to a process for producing a heat-conducting layer made from electrically conductive particles on a substrate. The process is characterized according to the invention in that the electrically conductive material is supplied to a heat-producing device in wire form and thus exposed to a temperature of  greater than 5,000xc2x0 K. The conductive particles thus produced are conveyed in air onto the substrate surface. It is thus important in the process of the invention that this application process is carried out in ambient atmosphere, that is in room air. Hence this process is simple and cost-effective to construct.
It is preferable in the process of the invention if the heat is produced by means of electric arc. The distance between the device and the substrate is thus preferably 5 to 50 cm, particularly preferably 12 to 25 cm. It should be emphasized in particular for the process of the invention that the particles may be transported using compressed air. As a result of this simple measure it is now possible to also influence the process by varying the pressure. The compressed air may be varied in the range from 2.8 to 7.5 atmospheres.
The process of the invention thus permits the layer thickness of the particles applied to be adjusted by varying the distance of the substrate from the heat-producing device and/or by varying the speed of the particles as well as by the wire feed and the level of the electric arc current.
The process of the invention offers wide-ranging advantages. According to the invention a separating layer should be produced on the substrate as a first process step for the strip-like embodiment. This separating layer is thus designed as regards its structure so that during application the heat-conducting layer obtains the required strip shape, for example meandering or helical.
Surprisingly, it has been shown that such an insulating separating layer may be produced very simply by applying a formulation containing a resin and a solvent. It has been shown that if such an insulating layer is applied to the substrate surface by an application process which is conventional per se, the particles supplied to the substrate by the evaporation method at the points at which the separating layer adheres to the substrate, penetration of the particles into the substrate surface is prevented. The particles landing thus fall off from these regions on which the separating layer is applied and only adhere, that is they penetrate into the surface of the substrate, at the points at which there is no separating layer. FIGS. 4 to 6 now show, by way of example on a glass substrate, an applied aluminum alloy in which a separating layer is also present. FIGS. 4 to 6 illustrate that at the points at which the separating layer has been applied by the subsequent application process, adhesion of the particles has not taken place. In the example of FIGS. 4 to 6, a marker pen having a resin formulation and dyestuffs was used as the separating layer.
This opens wide-ranging possibilities with regard to the layout of the heat-conducting layer to be applied. Using the process of the invention it is thus not only possible to vary the thickness and the strip width, but also the layout of the heat-conducting layer to be applied may be controlled specifically by very simple application of the separating layer on the substrate.
One crucial advantage of the process of the invention is that a polymer coating may be applied over the entire surface of the heat-conducting layer which then serves as chip protection, for thermal and electrical insulation as well as adhesive for fixing the mirror frame. It is obviously possible that a polymer coating may be applied by means of a simple process, for example by rolling, spreading or another conventional application process, particularly due to the rough surface produced by the process of the invention having the particle formation described above. A further advantage is that this polymer coating has excellent adhesion to the base, that is to the conducting layer and/or to the separating layer. A silane-modified plymer, in particular a silane-modified alkyd resin, is preferably used as the polymer coating. The advantage of this system consists in that it is self-curing in air. As a result of this measure very simple and cost-saving application of chip protection is possible which at the same time serves as insulation and optionally as adhesive for the mirror frame.
With regard to the selection of material for the particles to be applied and for the substrates, reference is made to the above statements for the heatable mirror. Accordingly, it is particularly preferable if a material pairing glass and aluminum and/or aluminum alloy is used for the particles. For the insulating separating layer it has thus been proved to be advantageous if a marker pen known per se from the state of the art, for example Eddingmarker(copyright), is used here.
As a result of the excellent control possibilities of the process with regard to layer thickness, strip width and the layout, the process of the invention described in more detail above is particularly suitable for producing heat-conducting layers for heatable mirrors in the motor vehicle industry[, as described in claims 1 to 16].